1. Field of the Invention
Aspects of the present invention relate to a stable high voltage power supply to prevent a current from overflowing therein during contact between a human body and a high voltage unit or during the application of an erroneous load, with regard to a conventional high alternating current (AC) voltage output method used for a non-impact laser printer or multifunctional device.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional high voltage power supply 100. Referring to FIG. 1, the conventional high voltage power supply 100 includes a controller 102 to output pulse width modulation (PWM) signals. First and second input units 104 and 112 receive the PWM signals PWM1, PWM2, and PWM3 from the controller 102. First and second comparison units 106 and 114 output comparison signals according to the PWM signals PWM1, PWM2, and PWM3. First and second switching units 108 and 116 each perform a switching operation according to the comparison signals. First and second transformers 110 and 118 transform output power according to the operations of the first and second switching units 108 and 116. A rectification unit 120 rectifies output power from the second transformer 118.
When the conventional high voltage power supply 100 receives PWM signals PWM1 and PWM2 from the controller 102 in order to output a high alternating current (AC) voltage, the PWM signal PWM1 is converted to a direct current (DC) level via an R/C circuit. The R/C circuit may be embodied as a low pass filter (LPF). The DC level is input to the first comparison unit 106 and compared with the PWM signal PWM2 in order to output a comparison signal. According to the comparison signal, the first switching unit 108 performs an on/off operation of a first transistor 109. Thus, the first switching unit 108 generates a pulse waveform having a Vcc electric potential. The pulse waveform is amplified by the first transformer 110 and is output as a high AC voltage to a final output end.
When the conventional high voltage power supply 100 receives a PWM 3 signal from the controller 102 in order to output a high DC voltage and the PWM 3 signal is low, a second transistor 113 of the second input unit 112 is turned on. At this point, an electric potential of a collector is input to a non-inversion end (+) of the second comparison unit 114. The Vcc electric potential distributed by resistance is input to an inversion end (−) of the second comparison unit 114. The second comparison unit 114 compares the Vcc electric potential with the electric potential of the collector that is input to the non-inversion end (+) of the second comparison unit 114 to output a comparison signal.
The second switching unit 116 performs a switching operation according to the comparison signal output by the second comparison unit 114. The second transformer 118 transforms power output by the switching operation of the second switching unit 116. The rectification unit 120 rectifies the transformed power to output the high DC voltage. The high DC voltage is applied to a second side of the first transformer 110. The high AC voltage that is a result of the transformation by the first transformer 110 and the high DC voltage provided from the rectification unit 120 overlap in the second side of the first transformer 110 and are output to a device 130. The device 130 may be, for example, a developer.
The stability standard of a high voltage power supply provides that when the high voltage power supply outputs a high voltage, current levels are limited so that electrical resistance equal to approximately the electrical resistance of human skin (e.g., 2 kΩ) is applied to an output end of the high voltage power supply to limit the amount of current flowing into the output end. For example, a DC is limited to 2 mA and an AC having a frequency is limited to 0.7 mA×the frequency.
However, a conventional circuit, which is a power supply that receives a PWM signal transmitted by a CPU and generates a high voltage, outputs a constant voltage regardless of a change in load. When a user opens a cover of the image forming device to take out the developer, the user is likely to be exposed to a high voltage terminal. Conventionally, a switch is installed in the cover to switch off the high voltage circuit when the cover is opened. However, when the cover is open, although an input voltage (24 V) of the high voltage circuit is switched off, the switch for opening the cover may be defective, or other abnormal errors may occur. In this case, the high voltage terminal is exposed to the user when the developer is taken out of the image forming device, which can cause an electric shock. As a result, the user may received an electric shock, and the developer of a high voltage output end may be grounded and shorted, which causes circuit damage.